This invention relates to a method of breeding new varieties of a first filial generation plant.
The term "Brassica oleracea" shall be understood to mean leaf vegetables having genome CC and containing Brassica oleracea L, for example, head cabbage, kale, Japanese ornament kale, brussels-sprouts, kohlrabi, cauliflower, broccoli, kairan and the like.
The term "Brassica genus" shall be understood to mean leaf vegetables having genome AA and/or BB, such as leaf vegetables listed in the following Table 1, that is, Brassica napus and the other 10 leaf vegetables, for example, Brassica nigra, Brassica alba and the like.
An allopolyploidy plant (genome CCAA or AACC) has heretofore been bred by Dr. Sadao Nishi et al belonging to the Department of Agriculture and Forestry, Vegetable Testing Plant by treating an interspecific hybrid (genome AC) composed of common cabbage (genome CC) and Chinese cabbage (genome AA) with colchicine etc. so as to increase twofold chromosome number thereof. The allopolyploidy plant thus bred is referred to as Hakuran.
The Hakuran thus bred has a number of drawbacks. In the first place, the Hakuran is insufficient in compact head formation and small in seed yield. Secondly, even though selection is repeatedly effected, it is difficult to improve its uniformity. Third, it is difficult to utilize the Hakuran in vegetable culture. Finally, the Hakuran is mostly self-incompatible.
Such self-incompatibility of the Hakuran is reported by Mr. Kuriyama in his article, "Study on Vegetables", published in 1976 by the Department of Agriculture and Forestry, Vegetable Testing Plant.
Attempts have been made to eliminate the self-incompatibility of the Hakuran by selection, but none of these attempts has led to a satisfactory solution which can be used in practice since the Hakuran is still difficult to make self-compatible and also difficult in seed production.
It is possible to effect seed production of a first filial generation plant, i.e., F.sub.1 hybrid obtained by crossing one Hakuran with another Hakuran. In this case, the Hakurans having bad uniformity are crossed with each other, and as a result, the F.sub.1 hybrid plant thus obtained becomes more degraded in uniformity, thus increasing difficulty in utilizing it in vegetable culture.
The reasons why the polyploid inclusive of the allopolyploid has a bad uniformity are as follows. The gene number of the polyploid is increased by a multiple "a" of the monoploid prior to polyploidy thereof in response to the chromosome number. Difficulty in improving uniformity by selection is proportional to an increase of the gene number.
There are natural allopolyploids such, for example, as Brassica napus (genome AACC, chromosome number 2n = 38) and Brassica juncea (genome AABB, chromosome number 2n = 36). These natural allopolyploids, however, have been subjected to selection for hundred years, so that these natural allopolyploids have a good uniformity and hence are self-compatible.
On the contrary, the Hakuran is a new plant which has been bred for only 10 years, so that it is inevitable that the Hakuran has a bad uniformity. As a result, it is difficult to quickly improve the uniformity of the Hakuran by selection. Thus, measures must be taken to cross the Hakuran with a plant whose chromosome number is smaller than that of the Hakuran so as to reduce the chromosome number of the Hakuran and to utilize as the plant to be crossed with the Hakuran, a line having a high purity, uniformity and least possible variation. In addition, it is desirous that the seed yield is more than 10% that of the conventional common cabbage or Chinese cabbage for the purpose of selling the seed.